(a) Technical Field
The present disclosure of invention relates to a thin film transistor display panel and to a transistor structure usable therein.
(b) Description of Related Technology
Thin film transistors (TFT's) are routinely used in flat or curved panel display devices such as liquid crystal display (LCD) ones, organic light emitting diode display (OLED) ones, electrophoretic displays, plasma displays, and so forth. Such display devices generally include an opposed pair of electric field generating electrodes configured for generating an electric field therebetween and one or more electro-optical active layers that are subject to the generated electric field. One or more optical characteristics of the electro-optical active layer(s) changes in response to the creation and/or strength of the generated electric field. In the liquid crystal display (LCD) devices for example, a liquid crystal material layer serves as the electro-optical active layer, and an orientation of the liquid crystal molecules is typically modulated by the generated electric field. In the organic light emitting diode (OLED) type displays an organic light emitting layer typically serves as the electro-optical active layer and light is emitted as a function of current induced by the generated electric field. In these and the various other types of TFT-using displays, one of the pair of electric field generating electrodes is typically connected to an electronic switching device that includes at least one thin film transistor (TFT) used for selectively applying a received electrical signal to at least one of the electrodes, where the electro-optical active layer converts the selectively applied electrical signal to an optics-affecting attribute and thus controls the display of a desired image.
More specifically, the typical flat or curved panel display includes a substrate on which one or more thin film transistors (TFT's) are integrally formed and electrical performance of the these integrally formed TFT's can greatly affect display performance. During mass production fabrication of the TFT's, multiple layers of electrode-forming conductors, of semiconductive material, and the like are photo-lithographically defined and shaped (patterned) in accordance with the photo-lithographical based patterning where at least one photo-lithography mask is generally used in the patterning process and one or more etch masks may also be used in the patterning process.
In the meantime, and as alluded to above, the semiconductive material that is used in formation of the TFT's plays a significant role in determining electrical characteristics of the formed thin film transistors. Amorphous silicon (Si-a) is one example of much used semiconductive material. However, a charge mobility attribute of Si-a tends to be undesirably low, so that it becomes difficult to mass produce high performance thin film transistors while using amorphous silicon (Si-a). On the other hand, when a more organized crystalline material such as polysilicon (Si-p) is used, charge mobility tends to be substantially higher, so that it becomes easier to mass produce high performance thin film transistors when using the more crystalline-wise organized polysilicon material. However, a cost of producing polysilicon tends to be high as compared to amorphous silicon (Si-a) and uniformity over large areas of the produced polysilicon (Si-p) may be low, so that there is a limitation in manufacturing a large thin film transistor display panel (for example for use in large area TV screens) while using only polysilicon (Si-p) for the entire area of the large-area TFT display panel.
Another type of semiconductive material that may be used (instead of Si-a or Si-p) comes from a class of oxides known as semiconductive oxides. Research has been ongoing on thin film transistors that use an appropriate one or more semiconductive oxides for attaining a higher charge mobility (higher than that of amorphous silicon), a higher ON/OFF ratio of current (better control over leakage current), a cost that is lower than that of polysilicon, and high uniformity of electrical characteristics when used in mass production of large area display panels.
It has been found that if a channel length in the thin film transistor using a semiconductive oxide (SOx) is short, there is a problem in that charge mobility deteriorates due for example to influence of a fringing field created by use of a high source bias. Moreover, during the mass production process, an upper portion of the SOx layer may be damaged when source/drain electrodes are etched by a corresponding one or more etch processes. When such damage occurs to the so-called, back surface of the channel portion of the SOx-based TFT, reliability of the thin film transistor display panel tends to deteriorate and becomes difficult to mass produce large area display panels of consistent quality while using semiconductive oxide based TFT's.
It is to be understood that this background of the technology section is intended to provide useful background for understanding the here disclosed technology and as such, the technology background section may include ideas, concepts or recognitions that were not part of what was known or appreciated by those skilled in the pertinent art prior to corresponding invention dates of subject matter disclosed herein.